1. Field of the Invention
The present invention relates to an activated sludge process bed for use in sewage treatement, in which water pollution materials such as BOD, COD contained in sewage water like industrial effluent, sewage water discharged from home, etc. are oxidatively decomposed and eliminated by micro-organisms (i.e., activated sludge) cultivated and bred on a carrier (i.e., cultivation bed).
2. Description of Prior Arts
Hitherto, a catalytic oxidation method is known as one of conventional sewage treatment methods in which water pollution materials such as BOD, COD contained in the sewage water discharged from factories, hotels, homes and the like are decomposed by micro-organisms. According to this known method, micro-organisms are implanted, cultivated and bred on a surface of a catalytic element of a certain design, and the catalytic element is arranged in an aerator to decompose BOD, etc. contained in sewage water.
In such conventional catalytic oxidation method using a catalytic element, however, there exist such problems that aeration control is difficult and that micro-organisms may sometimes excessively increase to the extent of clogging up a treatment equipment paralyzing the decomposition of BOD, etc.
In another catalytic oxidation method, in which a corrugated plate of a hard synthetic resin is used as a catalytic element, there exists such problems that it takes long for aerobic bacteria to be implanted and cultivated, that a film formed by bacteria is easy to be peeled due to water flow at the time of aeration or by some physical shock, and that some pieces of the peeled film come to float as scum while the other being precipitated into the bottom of a treatment tank as sludge declining sewage treatment performance thereby.
In a further method for sewage treatment in which a sponge piece is used as a catalytic element floating in an aeration tank, there exists a problem that the sponge piece tends to move to one corner of the tank due to water flow at the time of aeration or by wind, eventually inhibiting uniform treatment.
In view of the foregoing problems pertaining to the catalytic oxidation method, the applicant has proposed an activated sludge process bed as disclosed in Japanese laid open Patent Publication (unexamined) No. 59-145096. According to this prior activated sludge process bed, of which perspective view is shown in FIG. 2 and a partially enlarged view in FIG. 3, a mesh-like or grid-like outer peripheral surface of a cylindrical core 11 is wrapped with a porous member 12 at a specified thickness, e.g., 15 to 20 mm, said core 11 being composed of a hard synthetic resin resistant to corrosion over a long period of immersion in sewage water, both upper and lower part thereof being tightened with tightening rings 13, several parts of the cylindrical body being further tightened with tightening belts 14 so that the porous member 12 is tightly fastened to the cylindrical core 11. In these drawings, a reference numeral 15 is holes perforated on each tightening ring 13 to insert supporting rods for supporting the bed therethrough. In place of such holes, it is also preferred to attach supporting wires on the upper and lower end portions of the rings 13. The porous member 12 is formed of a lot of corrosion resistant vinylidene chloride yarns intertwisted one another or of other spongy, mesh-like or fibrous synthetic resin. Then a plurality of activated sludge process beds are vertically disposed in an aeration tank.
According to such prior activated sludge bed, following advantages are attained. That is, there is no possibility that the activated sludge process beds are moved by bubbles blown out of an aeration pipe, micro-organisms can be implanted and cultivated in rather short period, and the film of micro-organisms is not peeled at all. Furthermore, aerobic bacteria and anaerobic bacteria decompose BOD, COD, etc. further breeding themselves, while other giant micro-organisms are spontaneously bred or come to live at the center portion of the porous member and eat away these aerobic and anaerobic bacteria and autolize them. As a result, it is possible to prevent excessive increase of the aerobic bacteria so as not to produce further sludges. When a plurality of tanks are installed in a row and dissolved oxygen concentration (hereinafter referred to as "DO value") by the aeration is changed tank by tank as well as depending on whether inner periphery or outer periphery of the cylindrical activated sludge process bed, it is expected to perform the sewage treatment efficiently at a high BOD/COD elimination rate by means of respective beds, each having adequate breeding layers formed of aerobic and anaerobic bacterias while balancing nutritively the air, nitrogen and phosphorus they need according to the progress of the treatment.
The sewage treatment method disclosed in the above Publication No. 59-145096 has certainly various advantages as described above, but at the same time following disadvantages are to be pointed out. That is, as a matter of fact, from the viewpoint of operation control, it is quite difficult to control aerobic and anaerobic bacteria accurately so as to keep a correct and desired DO value tank by tank as well as inside and outside of each tank by merely changing the amount of air supplied from the aeration pipe since such other factor as water flow gives influence to the control. Besides, daily operation control in the treatment will require much labor. Accordingly, since the accurate control of DO value is difficult, it will be further difficult to eliminate BOD and COD at high efficiency. In particular, when BOD and/or COD is at lower level, the treatment will be difficult all the more.